High-temperature oxidation corrosion oxygen diffusion

At ordinary temperatures in neutral or near neutral water, dissolved oxygen is necessary for appreciable corrosion of iron. In air-saturated water, the initial corrosion rate may be high. This rate may diminish over a period of time as the iron oxide film is formed and acts as a barrier to oxygen diffusion. Since the diffusion rate at steady state is proportional to oxygen concentration, it follows that the corrosion rate of iron is also proportional to oxygen concentration. 

In characterizing HTC, X-Ray Diffraction is a very useful and reliable technique for indexing patterns of phases leading to phase identification in the corrosion product known as surface scale. In fact, this high temperature phenomenon is different from the mst layer formation (in the presence of moisture) since it virtually occurs in dry gaseous environments or molten salts, hi certain cases, HTC may be an internal oxidation process when oxygen diffusion is faster than the surface oxidation rate. 

Position of inert markers (e.g., Pt or AljOj partides) after oxidation when the scale growth is diffusion controlled (a) growth by outward metal cation diffusion, (b) growth by inward oxygen anion diffusion, and (c) growth by counterditfusion of both spedes. (From Kofstad, R, High Temperature Corrosion, Elsevier Applied Science, London, U.K., 1988.)... 

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